Simply put, he would not clear himself if he spoke up and told LE what they want to know, if he is guilty of ANY PART of Michelle's disappearance. Even if he was not directly involved but knows who is, he is not going to be totally innocent, there would be charges against him for accomplice, accessory after the fact, obstruction of justice, interference with an investigation, etc. So "manning up" and telling LE everything they want to know would not clear him, and that's why he's not talking.
Btw, there is a slight difference in a suspect telling LE what THEY want to hear, rather than telling them what they, the suspect, knows. LE can put pressure on people, feed them details, and get them to a point that they will confess to just about anything. It's happened. That's where lawyers come into play. LE's hands are tied now, they can't even talk to him without his lawyer present, so they are not likely to "beat" a confession out of him.
Smart guy.
MOO